26 SEPTEMBER 2014 • VOL 345 ISSUE 6204 1567 SCIENCE
sciencemag.org
(5). So far, nobody has been able to meet
all four requirements. The challenge is to
adapt photovoltaic (PV) systems capable
of achieving high efficiencies to drive the
water-splitting reactions in a scalable fashion. One of the primary difficulties is that
splitting water to H2 and O2 requires the application of 1.23 V (thermodynamics) plus
additional overpotentials associated with
driving the hydrogen evolution (HER) and
oxygen evolution (OER) half reactions:

4H2O + 4e; → 2H2 + 4OH; (HER)4OH; → O2 + 4e; + 2H2O (OER)

The HER and OER overpotentials are on
the order of 0.1 and 0.3 V, respectively; thus,
the overall water-splitting reaction requires
a total of ~1.7 V (6). This voltage constrains
the number of systems that are capable of
achieving high solar-to-hydrogen (STH)
conversion efficiencies and are not prohibitively expensive.

Mesoscopic and thin-film perovskite-based
PV systems have attracted interest because
they may break the cost-efficiency paradigm.
They are composed entirely of Earth-abundant elements, have low-cost manufacturing
processes, and are capable of achieving very
high solar-to-electricity power conversion efficiencies (η). The past 2 years have seen the
efficiency of methylammonium lead halide
perovskite PVs advance considerably, with reported devices producing η > 15% (7–9). An
important aspect of the high-power conversion efficiency is the high photovoltages that
are produced.

Luo et al. take advantage of these high-photovoltage perovskite PVs by configuring
two cells next to each other in electrical series
in order to efficiently split water. Integration
of solar cells in series produces a sum of the
photovoltages, whereas the current from each
cell is matched to the others. In this example,
the two cells each produce a short-circuit current density (Jsc) of more than 20 mA cm;2.
Because the cells are situated next to each
other, the current from each cell is constant,
but the electrode area effectively doubles,
producing half the current density (~10 mA
cm;2). Importantly, the open-circuit photovoltage (Voc) of each cell was more than 1 V,
thus providing a total of up to 2 V to drive the
water-splitting reactions. Another advance
reported by Luo et al. was the introduction
of a nickel/iron–layered double hydroxide
(NiFe LDH) catalyst deposited on a nickel
foam electrode, which can carry out both the
HER and OER reactions at 10 mA cm;2, with
overpotentials of 0.21 and 0.24 V, respectively.

As the two perovskite cells produce 10 mA
cm–2 under a load of 1.68 V, a STH efficiency
over 12% is achieved. Although the NiFe LDH
catalyst is not the best at the HER reaction,
the perovskites compensate with their high
photovoltage, and using a single catalyst material for both HER and OER reactions offers
a potential cost advantage.

While the 12% water-splitting efficiency
reported is already exceptional, there are
several paths to improvement. Use of a single
band-gap material in a tandem configuration is not ideal, and combining a perovskite
cell with a smaller band-gap semiconductor such as silicon could produce over 20%
STH efficiencies (6). Some loss in available
photovoltage by substituting a lower-volt-age silicon cell for one of the high-voltage
perovskite cells in order to increase the photocurrent may be compensated by the use of
a better HER catalyst that requires a smaller
overpotential. The NiFe LDH catalyst is also
opaque and not amenable to an integrated
photoelectrochemical system. It is not yet
clear if alternative transparent catalysts are
absolutely necessary or if the separated PV/
electrolyzer configuration used here will ultimately be viable (5).

There are very few reports of other sys-tems producing over 10% STH efficiencies.

In one notable example, a
GaInP2/GaAs tandem cell
with platinum catalysts
achieved a 12.4% STH water-splitting efficiency (10). This
high efficiency comes at a
high manufacturing cost of
the PV, however. In addition, the very rare elements
indium and platinum are
used, which prevent scal-ability. There are no previous examples of over 10%
STH from systems that have
used only Earth-abundant
elements and been driven
by potentially cheap PV.
The combination of high
STH efficiency with a cost-effective system composed
entirely of Earth-abundant
elements reported by Luo
et al. is thus unrivaled. They
have achieved three of the
four necessary criteria for a
practical water-splitting device. One major drawback of
this system is the instability
of the perovskite PVs, which
results in a degradation of
the photocurrent over a
period of hours. The cause
of the instability is not yet
fully understood, and this
issue clearly needs to be resolved if these
perovskite PVs are ever to be commercially
viable (11). In addition, use of appropriate
protection coatings, as has recently been
demonstrated for silicon solar cells, may
allow use of these perovskite cells in integrated solar water-splitting systems such as
the artificial leaf (12–14). It will be exciting
to see if perovskites will be the first to meet
all four criteria to win the solar hydrogen
race and beat out fossil fuels for our energy
future. ■